A work entitled "Space Telecommunications" in the Scientific and Technical Telecommunications Collection published by Masson, 1982, and in particular in Vol. I thereof at pages 92 to 95 and pages 259 to 261, describes firstly the grouping together of a plurality of antennas fed simultaneously by a single transmitter with interposed phase shifters and power dividers, such that the radiation characteristics of the group depend both on the radiation pattern of each antenna and on the distribution of power in phase and in amplitude. This property is made use of to obtain a radiation pattern that could not be obtained using a single radiating source. In addition, if the characteristics of the phase shifters and of the power dividers are changed by electronic means, it is possible to obtain a quasi-instantaneous change in the radiation pattern. The simplest form of grouping for radiation sources is an array in which all of the sources are identical and differ from one another by translation in some direction. In particular, it is possible to have arrays which are rectilinear or planar.
The above-mentioned document also describes the use of antennas having reflectors for generating multiple beams, having the advantage of low weight and the possibility of obtaining large radiating areas by using deployable structures. Antennas of this type are generally used when it is desired to generate numerous narrow beams. In general, the system for illuminating the reflector is off-center relative to the reflector so as to avoid masking any of its radiating aperture. Masking in the aperture gives rise to higher levels of secondary lobes which is to be avoided at all costs in this type of application. The main reflector may be a paraboloid, for example. The multiple beams are obtained by placing a set of illuminating sources in the vicinity of its focus, with each source corresponding to one of the beams. Since they cannot all be placed exactly at the focus, the illumination is not geometrically perfect and phase aberrations result which degrade radiation performance somewhat. The radiation pattern is deformed, with reduced gain relative to the value that can be obtained from the focus, and with parasitic secondary lobes. The degradation increases with increasing distance from the focus and with increasing curvature of the reflector. It is therefore necessary to make reflectors which are as "flat" as possible, i.e. having a high value for the ratio of focal length to aperture diameter. This gives rise to structures which are large in size and which present problems of accuracy and mechanical strength. In addition, mutual parasitic coupling may exist between the various sources, thereby giving rise to additional secondary lobes.
In space, applications that require the radiated wave to be electronically deflected over a wide field of view give rise to angular deviations of several beam widths. It is consequently essential to be able to control the shape of the antenna radiation pattern accurately. The configuration of these large antennas must also take account of several system aspects:
small volume on board the satellite, making it necessary to use the same antenna for transmitting and receiving simultaneously;
compatibility with the mechanical mounting facility on the platform, and on the launcher both before and during operation;
good temperature control; and
the possibility of having numerous missions and users.
The object of the invention is to solve these various problems.